PHYTOREMEDIATION OF ORGANIC AND INORGANIC ...

Asante-Badu et al.: Phytoremediation of organic and inorganic compounds in a natural and an agricultural environment: a review

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APPLIED ECOLOGY AND ENVIRONMENTAL RESEARCH 18(5):6875-6904.

http://www.aloki.hu ● ISSN 1589 1623 (Print) ● ISSN 1785 0037 (Online) DOI: http://dx.doi.org/10.15666/aeer/1805_68756904

© 2020, ALÖKI Kft., Budapest, Hungary

PHYTOREMEDIATION OF ORGANIC AND INORGANIC

COMPOUNDS IN A NATURAL AND AN AGRICULTURAL

ENVIRONMENT: A REVIEW

ASANTE-BADU, B.1,2 – KGORUTLA, L. E.1,2 – LI, S. S.1,2 – DANSO, P. O.3 – XUE, Z.1,2* – QIANG, G.1,2*

1College of Resources and Environmental Science, Jilin Agricultural University, 2888 Xincheng

Street, Changchun 130118, Jilin Province, PR China

2Key Laboratory of Sustainable Utilization of Soil Resources in the Commodity Grain Bases of

Jilin Province, Jilin Agricultural University, Changchun, PR China

(phone/fax: +86-0431-8453-2991)

3Key Laboratory of Groundwater Resources and Environment, Jilin University, Changchun, PR

China

*Corresponding authors

e-mail: [email protected] (Z. Xue); [email protected] (G. Qiang)

(Received 19th May 2020; accepted 13th Aug 2020)

Abstract. Phytoremediation is currently an area of trending research due to its huge potential as a

sustainable substitute for traditional methods of restoring contaminated sites. It is a profitable and

ecological alternative to mechanical and chemical remediation techniques used worldwide. An increase in

soil, water, and air pollution has severely disturbed an ecosystem functions and poses a huge threat to the

natural and agricultural environment as well as public health. Remediation of the contaminated

environment is one of the paramount concerns of the world. Hence this article deliberates on the general

problems of pollutants linked to phytoremediation techniques of organic and inorganic contaminants,

especially agrochemicals, petroleum, and explosive compounds. The paper also reviews a systematic

assessment of the recent progress in the phytoremediation of contaminants in a natural and agricultural

environment. Additionally, we highlight the benefits and limitations of phytoremediation along with a

brief clarification of the resilient mechanistic removal of contaminants by a three-phase method. Finally,

the perspective of biotechnological approaches in remediation is also suggested; taking into consideration

the future of synergistic remediation approaches and genetically improved plants to enhance

phytoremediation.

Keywords: hyperaccumulation, pollutant, phytoextraction, phytotransformation, phytovolatilization

Introduction

Phytoremediation is a term related to ecological restoration technology that takes

plants as the main source. Therefore, phytoremediation can be explained as using plants

(shrubs, trees, aquatic plants, and grasses) and associated microorganisms for the

elimination, degradation, or separation of contaminated sites in an environment (Ossai,

2019; Chirakkara, 2015; Bruneel, 2019). Different approaches have been employed to

remediate contaminated sites, but phytoremediation is well-thought-out as a possible

alternative, effective, ecologically friendly, and best likened to other outmoded

physicochemical methods (Burges et al., 2018; Rajput et al., 2019).

During the process of phytoremediation, compounds that can be remediated include

(i) organic waste (ii) metals (iii) inorganic substances (iv) agrochemicals (v)

metalloids (vi) radiochemical elements (vii) petroleum hydrocarbons (viii) explosives

and (ix) chlorinated solvents (Cristaldi et al., 2017; Misra, 2019; Abdel-Shafy and


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Mansour, 2018). The key sources of organic and inorganic pollutants occur naturally

but others are a result of the use of fuels, solvents, and pesticides. Several organic and

inorganic compounds are harmful and connected to health issues worldwide (Dixit et

al., 2015; Li et al., 2017). These pollutants are on the rise within the environment

through several activities in agriculture (pesticides, herbicides), industry (chemicals,

petrochemicals), spillage (fuel, solvents), wood processing, and military activities

(explosives, chemical weapons), etc. (Tripathi et al., 2020). Many contaminants

within the agricultural soils can cause a substantial amount of damage to plant growth,

soil ecological functions, and human health (Cai et al., 2019; Ngole-Jeme and Fantke,

2017). For instance, nitrobenzene impedes soybean seed growth and initiate

genotoxicity in the cells of its root tip (Guo et al., 2010). There are also varied

contaminants generally in soils or areas to be remediated, which contain trace

elements and organic chemicals. Even though this is still a herculean task, many

groups have faced action recently on contaminated soil phytoremediation (Gutiérrez-

Ginés et al., 2014; Xu et al., 2019).

The extreme pressures from organic and inorganic waste and the magnitude of

contamination have caused widespread concern as intentional or unintentional exposure

of these materials poses a major threat to the environment and public health (Cristaldi et

al., 2017; Rajput et al., 2019). Also, the events and mishaps of these contaminations

worldwide have drawn the attention of the general public. Progressively, more people

are alarmed and as a result, researchers are in search of the most effective remedies to

curb these pollutions. The Soil Pollution Prevention and Control action plan by

countries like China in 2016 was estimated to cost US$90 billion, aiming to utilize and

develop effective remediation methods (Hou and Li, 2017). Conversely, these

approaches do have some disadvantages such as high cost in operation and mobilizing

contaminated material to the site of treatment which intensifies the risk of secondary

contaminations.

Many techniques have evolved in the past decades intending to restore sanity to our

environment and a wide acknowledgment of these methods is on the basis that it is an

environmentally friendly method and a cheaper approach to get rid of contaminants.

Currently, the extent of studies that have employed phytoremediation techniques to

clean and reduce environmental pollution is increasing (Dixit et al., 2015; Rodriguez-

Narvaez et al., 2017). However, despite several reports by many authors on the issue of

remediation, there is still missing information regarding materials on the possible

antagonistic or synergistic effects of diverse contaminants and their cross-accumulation

or detoxification in both natural and agricultural environment.

Here, we:

• Provide an overview of phytoremediation of contaminants from agrochemicals

(pesticides/fertilizers), petroleum compounds, explosive compounds, and

inorganic compounds.

• Discuss the challenges in phytoremediation methods and also try to suggest an

improvement in the biotechnological approaches for future research

perspectives to enhance the phytoremediation of organic and inorganic

pollutants.

This paper, we believe will provide a critical and extensive review of studies in

natural and agricultural environment specifically in addressing the recent literature

regarding remediation of organic and inorganic compounds.


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Phytoremediation techniques

Phytoremediation has great promising as a natural on-site treatment with solar energy

capable of treating the land and a vast area of moderately contaminated sites.

Contrasting other kinds of remediation, plants grown for ornamental purposes in

gardens and landscaping projects have become an important tool in recent years (Liu et

al., 2018; Dodangeh et al., 2018; Cheng et al., 2017; Ranieri et al., 2016). Besides being

attractive to the environment, some of these ornamental plants accumulate or

decompose pollutants when they grow in soil polluted by heavy metals and organic

pollutants (Liu et al., 2018). Just like heavy metals, an organic pollutant in plants is

remediated by several natural biophysical and biochemical processes. Trees with large

roots and high transpiration rates, such as Populus (Populus spp.) or Willow (Salix spp.),

are well-known in the phytoremediation process (Srivastav et al., 2018). Such plants

have a multiplicity of effects on these toxic compounds. They can be fixed, stored,

volatilized, and converted to varying degrees (or even mineralized) or a combination of

the processes which are conditional on the particular compound, environmental

conditions, and plant genotypes. Phytoremediation remains a widely used method;

however, if a cautious selection of the best appropriate plants and proper agronomic

methods are not used appropriately, there could be a lack of control and plant

availability. The mechanism that plants use to promote remediation (Fig. 1) includes

plant extraction, plant degradation, plant stabilization, plant volatilization, and

rhizosphere decomposition.

Figure 1. Presentation of phytoremediation of organic and inorganic pollutants from a

contaminated environment (adapted from Favas et al., 2014; Avensblog, 2020)

Phytodegradation (phytotransformation)

Phytodegradation (also known as phytotransformation) involves the uptake of

organic contaminants to be decomposed (metabolized) or mineralized in plant cells by

certain enzymes. These include nitroreductase (through the decomposition of


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nitroaromatic compounds), dehalogenase (through the decomposition of chlorinated

solvents and pesticides), and volatile enzymes (involved in the decomposition of

aniline) (Kumar et al., 2018). Plant species such as Populus and Sage are illustrations of

plants with these enzyme systems (Leung et al., 2019; Feng et al., 2017). For instance,

Chen et al. (2016) reported that the Armoracia rusticana possess the ability to degrade

benzophenone within its tissues. These green plants are classified as the biosphere

‘green liver’. Also, a current study employed phytotransformation for organic

compounds like insecticides, chlorinated solvents, herbicides, inorganic nutrients, and

munitions (Kumar et al., 2018).

Phytostabilization (phytoimmobilization)

Phytostabilization or phytoimmobilization is an in situ technology whereby toxic

compounds are integrated into the root cell wall lignin to form non-toxic substances,

hence decreasing the existence of pollutants in the natural and agricultural environment

(Khalid et al., 2017; Mahar et al., 2016). Metals are precipitated in insoluble forms

under the direct action of root exudate and subsequently enter the soil medium. The

main challenge relating to this technique is the avoidance of the concentration of

pollutants and suppressing their dispersion in soil (Favas et al., 2014; Ali et al., 2013;

Fan et al., 2017). Studies conducted by Khalid and coworkers (Khalid et al., 2017) have

referenced a great number of examples of plants grown for this purpose which are but

not limited to species of the genera Haumaniastrum, Eragrostis, Ascoliepis, Gladiolus,

and Alyssum. These plant species can also contribute to phytostabilization owing to their

capability to release a greater number of chelating agents. Other research studies

(Lebrun et al., 2018) showed that in phytoimmobilization, unclean soil is covered with

vegetation that is resistant to high applications of harmful elements, thereby limiting

soil erosion and the leaching of pollutants within groundwater. These substances

immobilize pollutants, inhibiting their uptake and reducing their mobility in soil. As a

result, plants with stabilizing potential play an essential role in polluted areas of

agricultural production and vegetation restoration (Eskander and Saleh, 2017; Saha et

al., 2017). It is reported (Eskander and Saleh, 2017), that these substances were known

to immobilize pollutants, prevent absorption, and thereby decrease their movement in

the soil. This accounts for why plants with phytostabilization potential are very valuable

for mining, waste, and vegetation restoration in contaminated sites. Similarly, Yadav

and colleagues (Yadav et al., 2018) have also revealed that phytostabilization is very

useful in removing inorganic contaminants, from soil contaminated with Arsenic (As),

Copper (Cu), Lead (Pb), Zinc (Zn), Cadmium (Cd), Chromium (Cr) and other metals in

their sediments.

Phytovolatilization

Phytovolatilization is the capability of some green plants to engross certain

metals/metalloids and later release them through vaporization. Some ions groups like

IIB, VA, and VIA in the periodic table (especially mercury (Hg), selenium (Se), and

As) are being taken by the roots, transformed into non-toxic forms and later volatilized

into the atmosphere (Limmer and Burken, 2016). Conversely, the disadvantage of

phytovolatilization lies in the fact that toxic compounds discharged into the atmosphere

can undergo precipitation and re-deposit back into the environment, as this will give rise

to other toxic substances (Nikolić and Stevović, 2015). Examples of plant species from


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studies (Ali et al., 2013; Mani and Kumar, 2014) include Stanleya pinnata and

Astragalus bisulcatus or transgenic plants (with bacterial genes) such as Liriodendron

tulipifera, Brassica napus, Arabidopsis thaliana, or Nicotiana tabacum. This method

can also be applied to tackle organic compounds and other heavy metals like Se and Hg.

Phytoextraction (phytoaccumulation, phytoabsorption or phytosequestration)

This mechanism refers to the absorption of pollutants by roots, followed by

displacement and accumulation in air particles. Phytoextraction technology as reviewed

by Parmar et al. (2015) involves the eradication of contaminants from soil, groundwater,

or surface water by living plants. This technique according to research by Sarwar et al.

(2017) is mainly engaged in the remediation of soils polluted by metals (Cd, Nickel

(Ni), Cu, Zn, Pb), however, it could also be used for other elements (such as Se, As) and

organic compounds. This method favors hyperaccumulator species (which will be

discussed in this paper) that can maintain a great absorption of exact metals in their air

portions (0.01-1% dry weight, depending on the metal) (Parmar, 2015).

Phytoaccumulation, also called phytoextraction or hyperaccumulation, reported by Xiao

et al. (2017) uses cationic pumps and sorption to remove metals, salts, and organic

compounds from the soil by absorbing water available to plants. The process

commences by sowing the metal-accumulating plant in metal-polluted soil and coupled

with extensive agricultural practices. Many authors (Van der Ent et al., 2013; Xiao et

al., 2017; Reeves et al., 2018) have proposed Elsholtzia splendens, Alyssum bertolonii,

Thlaspi caerulescens and Pteris vittata as known examples of hyperaccumulating plants

of Cu, Ni, Zn/Cd, and As respectively. Several plant species near metal mining areas as

stated by Saxena et al. (2020) thrived in soils heavily contaminated with metals, hence

for remediation of contaminated areas, it is necessary to select the kind of plants which

can absorb and transport metals in various conditions. However, some soils are highly

contaminated hence the elimination of metals through this method will save an

uncertain amount of time.

Phytofiltration

This technique uses plants to absorb distillate and/or precipitate contaminants,

especially radioactive elements, and heavy metals, from the environment through the

root structure or further submerged organs. In this process, Parmer and Singh (Parmar,

2015) reviewed that plants are stored in a hydroponic system through which wastewater

passes and is “filtered” by the roots (rhizofiltration) or other structures that can absorb

and concentrate pollutants (hyperaccumulators) or tolerate pollutants for the best results.

Phytofiltration remediates metallic substances like Cu, Ni, vanadium (V), Cr, Pb,

including other radionuclides such as caesium (Cs), strontium (Sr) and uranium (U).

Nevena et al. (Cule et al., 2016) in an experiment used the ornamental plant (Canabis

indica) to remediate Pb in wastewater. Their findings showed that the removal rate was

81.16% higher. The authors (Cule et al., 2016) supported the idea that terrestrial plants

are most appropriate for rhizofiltration than aquatic plants and that C. indica is best used

in rhizofiltration methods or floating islands for treatment of water polluted with Pb.

Zhang et al. (2005) researched on the efficiency of Cu removal from contaminated

water by Elsholtzia argyi and Elsholtzi splendens in hydroponics. Their results

demonstrate that Elsholtzia argyi showed better Cu phytofiltration (removal rate of 50-

90%) than Elsholtzi splendens (removal rate of 45-80%), which was linked with better


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capability to higher Cu concentrations and translocation to shoots. One could infer that

various physicochemical properties of plant species are factors in the choice of plant for

phytofiltration. This technology is capable of dealing with industrial discharge,

agricultural runoff, and mine drainage. Several authors have indicated some promising

examples of such plants as Brassica juncea, Helianthus annus, Fontinalis antipyretica,

Phragmites australis, and some species of Salix, Populus, Lemna, and Callitriche (Van

der Ent et al., 2013; Bonanno and Cirelli, 2017; Favas and Pratas, 2016; Tatar et al.,

2019).

Rhizodegradation (phytostimulation)

Rhizodegradation or phytostimulation is the breakdown of organic compounds in the

soil through the microbial activity of the root (rhizoshere) (Echereme, 2018). This is

improved biodegradation of contaminants by a particular plant species root-associated

fungi and bacteria (Ali et al., 2013; Khalid et al., 2017). There are free-living, symbiont

mycorrhizal fungi that are associated with plant roots and are significantly more

beneficial for the production and biochemical availability of nutrients such as cobalt

(Co), Cu, Zn, Ni, nitrogen (N), phosphorus (P), potassium (K), sulphur (S) and Calcium

(Ca) through the extensive hyphal network (Sarwar et al., 2017). In research by Jiang et

al. (2017), it was demonstrated that bacterial species, for instance, Pseudomonas,

Acinetobacter, Bacillus, and Cupriavidus improved their environmental adaptability and

was resistant to Pb, Cd, and Cu in rhizospheric soil around the plant. The authors

deduced that Boehmeria nivea L. (known to be evolving around chemical refineries)

was improved by increased concentrations of the particular microorganisms (Jiang et

al., 2017). Also, the plants themselves can release biodegradable enzymes. The

microbial association in the rhizosphere is heterogeneous due to the changing spatial

distribution of nutrients, but species of the genus Pseudomonas are the organisms

largely connected with the root (Ali et al., 2013; Singh and Singh, 2016; Salem et al.,

2018). The death of plants ought to be considered and agronomic methods must be used

to reduce it by timely planting in the growing period, digging a hole in diameter, and

feasibly filling it with unpolluted plant soil, for better survival of trees, thus bringing a

greater efficiency of phytoremediation.

Phytodesalination

According to literature, this is a recently informed remediation approach that

removes saline from internal salts. Studies have reveals that the efficacy of Suaeda

maritima and Sesuvium portulacastrum in eradicating and accumulating sodium

chloride (NaCl) from highly salty soil, has demonstrated to be very effective (Ali et al.,

2013; Kumar et al., 2019b). Phytodesalination may possibly occur in parallel with

phytoremediation of heavy metal contaminated soils in arid regions, increasing the

prospect of this process. Halophytes (salt-tolerant plants) have been suggested

(Padmavathiamma, 2014) to naturally adjust to cope with environmental stresses, such

as heavy metals and other organic contaminants (Padmavathiamma, 2014).

Iniyalakshimi (Iniyalakshimi, 2019) also found that S. portulacastrum, which has high

sodium and chloride absorption ability may be a possible candidate for reducing

secondary salinization due to irrigation with sodium-rich industrial wastewater (Fan et

al., 2019). Taken together, this indicates that S. portulacastrum (SpSOS1 and SpAHA1)

coordinates to alleviate salt toxicity by cumulating the efficiency of Na + extrusion to


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maintain K + homeostasis and defend the plasma membrane from oxidative harm

induced by salt stress. In a current study (Lastiri-Hernández et al., 2020), the capacity of

halophytes species namely; Bacopa monnieri (L.), Sesuvium verrucosum Raf. and

Wettst was assessed to increase their chemical properties in saline soil for 240 days in a

field. It was also reported that the association of these plants has a phytodesalination

ability of 1.21 t Naþ ha1 and this served to prepare the conditions for the crop growth

(Lastiri-Hernández et al., 2020). This technology involves an enzymatic breakdown of

pollutants, relevant for soil, groundwater, or surface water, and sediment sludges. This

process is an impressive kind from phytoextraction, even though it has its drawbacks.

Some of these drawbacks could include the duration of the process, the properties of the

soil (salinity, sodicity and porosity), the number and the initial weight of the plant

species.

Phytoremediation of organic contaminants

Currently, phytoremediation is offered as a cost-effective method to remove many

kinds of organic contaminants such as petroleum products, aromatic hydrocarbons

(BTEX), chlorinated solvents, explosives, and cyanides as aforementioned (Hattab-

Hambli et al., 2020).

Furthermore, one of the largest environmental disasters was in the Gulf of Mexico,

which had a major impact on the ecosystem and human health (Sandifer et al., 2017;

Singleton et al., 2016; Schaum et al., 2010; Han and Clement, 2018; Babcock-Adams et

al., 2017; Eklund et al., 2019). Crude oil kills small invertebrates, destroying soil

biological life thereby rendering such polluted soils habitable to only anaerobic bacteria.

Phytoremediation, as a natural, solar energy-driven in situ method, has great potential

for treating soils and large areas of moderately contaminated areas. Plants are carefully

selected and appropriate agronomic methods are used to properly manage

phytoremediation of organic pollutants (Schwitzguébel, 2017). Phytoremediation can

restore, balance a stressful environment due to the natural, synergistic relationships

between plants, microorganisms, and the environment. Further, during the in-situ

phytoremediation process, organic matter, nutrients, and oxygen are added to the soil

through the metabolic processes of plants and microorganisms, thereby improving the

quality and texture of the clean area (Schwitzguébel, 2017; Wiszniewska et al., 2016).

Phytoremediation of agrochemicals

The predominant misuse of agrochemicals over several years has polluted the

agricultural and natural environment. These chemicals cause various damages to

agricultural lands, various water bodies, and other land-dwelling organisms.

Various organic compounds (OCs), for example, organochlorine pesticides (OCPs)

and other agrochemicals are extremely toxic and persistent in the environment (Sun et

al., 2018).

The potential use of alfalfa, tomato, sunflower, and soybean species to remove

endosulfan from the soil was studied by Mitton et al. (2016). Their research results

showed that, except soybean, the phytoextraction rate of all other species at 60 days

increased with the decrease of soil pesticide levels. Sunflower plants had the highest

phytoextraction rate (2.23%), followed by tomato (1.18%), soybean (0.43%), and alfalfa

(0.11%). In additional research (Zhao, 2018), the dissemination of dichloro diphenyl

trichloroethane (DDT) residues in agricultural soils in southwestern Ontario, Canada,


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was also studied to determine the degree of degradation of tomato plants. The DDT

concentration in tomato fruits was 211.75 ug/kg, which was lower than the prescribed

value. Consequently, tomato fruits are eligible and valued, and may be a potential

choice for future field research. In a study (Qu et al., 2017) involving atrazine-

contaminated lake sediments, spiked watermilfoil (Myriophyllum spicatum), and curled

algae (Potamogeton crispus) were opened to 0.10 mg atrazine kg-1 soil at a decreased

rate of 76.15% and 75.65% respectively in comparison to Atrazine’s decline of 46.3%

non-cultivated soil.

To correct these contaminations, researchers have essentially used consortia of

various bacterial strains, such as atrazine and deisopropylrazine (Fan and Song, 2014)

via rhizodegradation/phytostimulation of agrochemicals from agricultural environment.

From a biotechnological point of view, bacterial strains can also be used to decompose

certain pesticides, such as the one Myresiotis et al. (2012) used to remediate the soil

contaminated with hydrochloride, metribuzin, acidobenzolar-S-methyl, propamocarb,

thiamethoxam and napropamide. Phytoremediation of pesticides is affected by some

factors. Their low bioavailability in soils might confine the attainment of this

technology.

Large numbers of plants have been tested to proficiently accumulate pesticides and

some of these plants are listed in Table 1. The capacity of these plants to uptake

pesticide residues varies greatly between plant species (Handford et al., 2015; Romeh,

2015). These plants are widely used because of their significant role in agriculture and

gardening and as a result of these good prospects for the accumulation of innumerable

organic pollutants, they are therefore generally considered phytoremediation plants

(Singh and Singh, 2017).

Significant interactions can occur between different pollutants in a polluted

environment. When there is soil-to-plants absorption, the accumulation potential is

affected by various plant properties such as water absorption potential and root

depth/structure (Eevers et al., 2017). When pesticides are trapped in plant root tissues,

they can be immobilized in the roots or transferred to an aerial portion of the plant

where the analytes can be stored, metabolized, or evaporated (Eevers et al., 2017). In

general, the objective of effective phytoremediation is not only phytoaccumulation but

also the degradation of pollutants in plant tissues as this can be done with or without

endophytic bacteria.

Phytoremediation of explosive compounds

After World War II, the commissioning and removal of weapons at military factories

brought about the pollution of the agricultural environment and other biological systems

with explosives like 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1, 3,5 triazine

(RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazocin (HMX) (Taylor, 2017;

Srivastava, 2015; Sheehan et al., 2020; Aguero and Terreux, 2019). These compounds

are the most widely distributed organic explosive contaminates found in nature. The use

of explosives during military exercises and operations leads to large-scale

environmental pollution, in which case the ecological balance is disturbed (Ndibe et al.,

2018). As a consequence of their manufacturing and disposal practices, these explosives

with their transformation products are major pollutants in soils, ground and surface

waters worldwide. For instance, ammunition contamination in the 1920s in Verdun,

France, still posed a danger to public health (Rylott and Bruce, 2019; Gorecki et al.,

2017).


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Table 1. Phytoremediation of pesticide by certain plant species from previous studies

Plant species Agrochemical Formula

Molecular

weight

(g/mol)

References

Castor bean/castor oil plant (Ricinus

communis)

Aldrin, Cl2H5Cl6 364.92

Rissato et al., 2015

Chlordane, Chlorpyrifos, C10H6Cl8 409.78

Dichlorodiphenyldichloroethylene (DDE) C9H11Cl3NO3PS 350.59

Diclofop methyl, Dllieldrin Cl4H5Cl4 318.03

Endrin C16H14Cl2O4 341.2

Hexachloro-cycohexane (HCH) Cl2H5Cl6O 380.91

Heptachlor, methoxychlor Cl2H, Cl6O 380.91

C6H6Cl6 290.83

C1OH5Cl7 373.32

DDT Cl4H9Cl5 354.49 Huang et al., 2011; Rissato et al., 2015

Maize/corn (Zea mays)

Dichloro diphenyl dichloroethane (DDD) Cl4HlOC14 320.05 Bogdevich and

Cadocinicov, 2010 DDE Cl4H5Cl4 318.03

DDT Cl4H9Cl5 354.49

Endosulfan C9H6Cl6O5S 406.92 Mukherjee and Kumar,

2012

Endosulfan sulphate C9H6Cl6O5S 406.92 Somtrakoon et al., 2014;

Somtrakoon, 2014

Hexachloro-cycohexane (HCHs) C6H6Cl6 290.83

Alvarez et al., 2015;

Bogdevich and

Cadocinicov, 2010

Chinese violet cress

(Orychophragmus

violaceus L. O. E. Schulz)

DDD Cl4HlOC14 320.05

Sun et al., 2015 DDE Cl4H5Cl4 318.03

DDT Cl4H9Cl5 354.49

HCHs C6H6Cl6 290.83

Common sunflower

(Helianthus annus)

Azoxystrobin C22H17N3O5 403.4 Romeh, 2015

DDD, DDE,

DDT

Cl4HlOC14 Cl4H5Cl4

Cl4H9Cl5

320.05 318.03

354.49

Mitton et al., 2014

Common yarrow

(Achillea millefolium) DDT Cl4H9Cl5 354.49 Moklyachuk et al., 2012

Sweet flag – Calamus

(Acorus calamus) Atrazine C8H14ClN5 215.68 Wang et al., 2012

Welsh onion/bunching

onion/long green

onion/Japanese

bunching onion/spring

onion (Allium fistulosum)

Phoxim C12H15N2O3PS 298.3 Wang et al., 2011

Love-lies-bleeding

(Amaranthus caudate) Glyphosate C3H8NO5P 169.07 Al-Arfaj et al., 2013

Field mustard/Turnip

rape/bird rape/Keblock (Brassica campestris)

Endosulfan C9H6Cl6O3S 406.92 Mukherjee and Kumar,

2012

Broadleaf

plantain/white man’s foot/Greater plantain

(Plantago major)

Azoxystrobin C22H17N3O5 403.4 Romeh, 2015

Chlorpyrifos C9H11Cl3NO3PS 350.59 Romeh and Hendawi,

2013

Cyanophos C9H10NO3PS 243.22 Romeh, 2014a,b

Source (common English name): https://en.wikipedia.org/wiki/ (accessed on April 7, 2020)

Explosives are sensitive to a xenobiotic, and their survival in an environment is

dangerous for living organisms; as they can migrate through aboveground soils,


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polluting groundwater (Taylor, 2017). Despite the devastating effects of pollution on

environmental and human health, the global market for explosives was forecast to grow

from $ 23.8 billion in 2017 to $ 31.2 billion by 2022, therefore lucrative, clean-up

methods are required instantly (BCC, 2018). There are innumerable practices connected

with the conversion of explosive pollutants by plants. RDX has been described

(Hannink et al., 2002) to have minor toxicity than TNT. Additionally, it can translocate

within plants, and consequently, these compounds can be stored in various parts of the

plants.

In the USA (Virginia Commonwealth University), a study by Via et al. (2016) was

conducted to investigate the impacts of explosives polluted soils using ecological

metrics in an experimental minefield on vegetative communities. Their results showed

that RDX and TNT contaminated plots had shifted in dominant functional traits,

suggesting an influx of more tolerant species as a result of new tolerant species filling

open niches in contaminated plots (Via et al., 2016). In another study (Kiiskila et al.,

2015), many agricultural and ornamental important plants were examined for their

capability to remediate TNT and RDX in barley (Hordeum sativum), Soybean (Glycine

max), alfalfa (M. sativa), chickpea (Cicer arietinum), maize (Zea mays), sunflower

(Helianthus annuus), pea (Pisum sativum), and ryegrass (Lolium multiflorum) species.

Furthermore, Das (2017) reported the two most effective species for TNT uptake-

Eurasian watermilfoil, Myriophyllum spicatum, and vetiver grass as well as

Chrysopogon zizanioides. For RDX phytoremediation, reed canary grass, rice, and fox

sedge showed good promise, although degradation of RDX in the plant tissue is limited

(Kiiskila et al., 2015). Conversely, there are limitations to this new technology. It is

only effective when treating shallow soils, ground, and surface waters. Plants can also

efficiently remove pollutants only close to the root zone since phytotoxicity is also a

drawback to this methodology.

Phytoremediation of petroleum compounds

Economic growth and industrialization have led to increased emissions of petroleum

hydrocarbons (PHC) and trace elements (TE). Due to their tenacity in an environment

and various toxicological effects on living things, they are well-thought-out to be the

greatest toxic pollutants in the world (Marchand, 2018). Most chemical contaminated

soils are linked with the release of petroleum products into the environment. The

intensification of global oil and gas activities, including oil exploration, drilling,

production, land storage, and transportation has also increased the menace of crude oil

spills and outflows (Okotie et al., 2018). Due to health problems, these water bodies and

land contaminated with crude oil are often not suitable for domestic and agricultural

uses (Tang and Angela, 2019a).

However, the exploration of crude oil has brought economic development, especially

in developing countries (like Ghana, Nigeria, Niger etc.), but the natural and

agricultural environment in these countries has also been damaged by the negative

effects of these oil industries (Ngene et al., 2016). For instance, water resources in the

Niger Delta are no longer fit for human drinking, but oil exploration in Nigeria cannot

be stopped as 90% of Nigerian foreign currency earnings come from crude oil

exploration (Okotie et al., 2018; Adekola et al., 2017; Siakwah, 2018). In addition to the

detrimental effects of the oil industry in our environment, various reports (Ramirez et

al., 2017) points out its negative impacts on human health as well. These are

psychological problems initiated by crude oil leakage, respiratory tract irritation, and


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blood disorders (Ramirez et al., 2017; Taheri et al., 2018). Sylvia reported that

contaminated sites pose a risk to human life owing to severe health complications

caused by unreceptive health effects from introduction to oil-soil contamination

(Adipah, 2019).

Phytoremediation is said to be a slowly developing process that can clean the

environment for a long time. This method is also influenced by external parameters,

including type and leaching of pollutants, soil chemistry, and photosynthesis (Lim et al.,

2016) and water-related effects on conservation and weather conditions. Exposure of

plants to organisms and pollutants for a longer period reduces their ability to uptake

contaminants (Zabbey et al., 2017). In current research by Viesser et al. (2020), three

bacteria were isolated namely; Bacillus thurigiensis, Bacillus pumilus, and Rhodococcus

hoagie. These were able to use petroleum hydrocarbons as the sole carbon during

vitrodegradation assays. The authors found that R. hoagii had the highest efficacy of

petroleum consumption, attaining 87% of degradation after only 24 h of cultivation.

Another experiment (Heidari et al., 2018) was conducted to study the effect of oil-

contaminated soil on Echinacea purpurea with four concentrations of crude oil. The

results depicted that this plant has a possibility for removing TPHs, up to 45.5% at 1%

crude oil contamination. The authors further reported that E. purpurea is a widely -

spread species that can be commendably used for phytoremediation of ≤ 10000 mg kg-1

crude oil-contaminated soil. Other studies suggested that the faster-growing flora (e.g.

grass species) are plants that effectively restore polycyclic aromatic hydrocarbons

(PAH) in contaminated soil (Srivastav et al., 2018; Kumar et al., 2019a). The adsorption

properties of TPH ought to be investigated as it aids to reduce the concentration of

organic material in the soil. Thorough remedial studies have to be carried out not only

to assess the effectiveness of the repair but also to investigate and implement the

potential for secondary pollutants.

These studies demonstrate that agricultural lands with low rates of oil contamination

allow the growth of plants. Furthermore, previous studies in phytoremediation of

contaminants have not focused so much on petroleum hydrocarbons rather all attention

has been on heavy metals remediation (Tang and Angela, 2019b), hence further studies

must be focused on this area.

Inorganic contaminants

Inorganic materials belong to man-made environmental activities, for instance,

smelting, mine drainage, chemical, and metallurgical processes, and also natural

processes. These sources release inorganic pollutants usually in the form of minerals

such as metals, salts, and natural substances (Masindi and Muedi, 2018; Fayiga et al.,

2018). Heavy metal contaminants are usually brought about by human activities, but

natural and biological contaminations are also common such as erosion and volcanic

activity, mining pollution over time; which causes toxic particles release in vegetation

nutrients and forest (Banunle, 2018). These contaminants are poisonous since they

sometimes accumulate in the food chain (Masindi and Muedi, 2018). Pb, Co, Cd and

other toxic heavy metals cannot be biodegraded, so they can be distinguished from other

pollutants, but they can accumulate in plants, even if the concentration is relatively low,

it can cause various diseases and diseases. Higher levels of essential and non-essential

heavy metals in the soil may inhibit plant growth and cause toxic symptoms in most


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plants (Ochonogor, 2014). Several remediation techniques for inorganic compounds are

known in literature but phytoremediation stands out.

Phytoremediation of inorganic contamination

Phytoremediation of inorganic contaminants comprises three technologies, plant

extraction (similarly known as phytoextraction/phytoaccumulation), root filtration

(rhizofiltration), and plant stabilization (phytostabilization) (Chirakkara et al., 2016;

DalCorso et al., 2019). In phytoextraction as aforementioned, heavy metals within the

soil are absorbed by the roots of the plants, transferred to the ground part, and

accumulated in the soil. The contaminants do not decompose, but remain in roots and/or

plant tissue (Chandra et al., 2017). After harvesting plant parts containing inorganic

contaminants, it is advisable to keep them in a safe place for discarding. It is reported

according to some authors (Banunle, 2018; Sharma, 2018) that the amount of

contaminated plant material treated is relatively small compared to the amount of

contaminated soil treated with in situ remediation techniques. There are differences in

rhizofiltration from the phytoaccumulation, as this is applied to contaminants dissolved

in shallow, underground, or wastewater (Chirakkara et al., 2016).

In the process of rhizofiltration of plant root, inorganic contaminants will be

adsorbed in the roots or deposited on the roots (Makombe, 2018; Makombe and Gwisai,

2018). The fixation of pollutants in the root zone by the effects of soil chemistry,

microbiology, and physics are also referred to as phytostabilization (Touceda-González

et al., 2017). Some soils are so severely contaminated hence metals removal using

plants would take an impractical expanse of time. Therefore, the normal practice is to

select drought-resistant, fast-growing fodder or plants which will be able to grow in

metal-contaminated and nutrient-deficient soils.

Hyperaccumulation process by some plant species

The remediation techniques noted here is plant extraction (phytoextraction),

identified as phytoaccumulation, where residues of pollutants are taking by plant roots

and transferred to other parts of the plant. Some metals are more deadly than others (e.g.

arsenic, cadmium is less toxic) for instance cobalt, nickel, chromium, mercury, and

selenium are very toxic even in small quantities (Masindi and Muedi, 2018). Plant

extraction according to Rascio and Navari-Izzo (2011) can be accomplished by

removing high concentrated contaminants from the soil, such as using

hyperaccumulators; uptake of low concentrations of extracts while maintaining high

growth status as in Populas sp. (Nissim et al., 2018). Subsequently, phytostabilization

removes contaminants and reduces their leaching from the soil by reducing crop roots.

Roots can also be an important material for transforming harmful metals into low-toxic

forms (Ali et al., 2013). Additionally, damage to plants depends on the microorganisms

that come with the root system and the enzymes that are secreted by the root to clear the

germs and then remove them by absorption and transpiration.

The roots that accumulate higher concentrations of iron in each tissue are higher; and

so, their habitat is acknowledged as hyperaccumulators (Srivastav et al., 2018). Qatari

plants reported in recent studies by Al-Thani (2019) are the most candidates for active

phytoremediation of contaminated compounds of such nature. Considering that

monitoring is employed, Phragmites australis, Typha domingensis, Amaranthus spp.,

Nerium oleander, and Ricinus communis are noted as important species of plant for


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successful ecological remediating and conserving a healthy environment (Al-Thani,

2019). Table 2 summarizes some examples of plant species and their metal

accumulation capabilities that have been tested for phytoremediation studies. In general,

different methods of phytoremediation have potent characteristics which make them

suitable for diverse soil and water pollution remediation. These methods of study are

delimited by duration, nonetheless it supports the capability of selected

phytoremediation plants to be considered (Tang and Angela, 2019a). For

phytoremediation through phytoaccumulation, the shoot must be obtained after uptake.

The shoot can be burned or otherwise destroyed (Valipour et al., 2015).

Table 2. Some plant species used in the hyperaccumulation of metals and their accumulation

capacity

Plant species Family Polluted

medium Metals

Metal accumulation

capacity

(mg kg-1DW)

Phytoremediation

mechanism and metal

accumulation compartment

References

Blue stool (Noccaea caerulescens)

Brassicaceae Water Pb 1,700–2,300 Rhizofiltration (aerial

parts/root) Dinh et al., 2018

Hemp (Cannabis sativa L.)

Cannabaceae Soil Cd 151 Phytoextraction

(aboveground plant parts Ahmad et al., 2016

Madwort (Alyssum markgrafii)

Brassicaceae Soil Ni 4,038 Phytoextraction

(aboveground plant parts) Salihaj et al., 2018

Hemp (Cannabis sativa L.)

Cannabaceae Soil Cu 1,530 Phytoextraction

(aboveground plant parts) Ahmad et al., 2016

Southern cone marigold (Tagetes minuta)

Asteraceae Water As 380.5 Phytoextraction (shoots) Salazar, 2014

Johnson grass (Sorghum halepense L.)

Poaceae Soil Pb 1,406.80 Phytostabilization (reduction

in rhizosphere) Salazar, 2014

Water/red birch (Betula occidentalis)

Betulaceae Soil Pb 1,000 Phytoextraction (shoots) Koptsik, 2014

Alpine pennygrass (Thlaspi caerulescens)

Brassicaceae Soil Cd 5,000 Phytoextraction (shoots) Koptsik, 2014

Sunflower (Helianthus annuus)

Asteraceae Soil Pb 5,600 Phytoextraction (shoots) Koptsik, 2014

Alpine pennygrass (Thlaspi caerulescens)

Brassicaceae Soil Ni 16,200 Phytoextraction (shoots) Koptsik, 2014

Black mustard (Brassica nigra)

Brassicaceae Soil Pb 9,400 Phytoextraction (shoots) Koptsik, 2014

Alfalfa/lucerne (Medicago sativa)

Fabaceae Soil Pb 43,300 Phytoextraction (shoots) Koptsik, 2014

Benefits and limitations of phytoremediation technology

Phytoremediation does not only have advantages but also disadvantages that need

to be considered when using this technique. Economically it is cost advantageous, but

tracking results can be time-consuming and more of these are elaborated in Table 3.

Choosing the technique that can recover multiple pollutants at once is tough

(Srivastav et al., 2018). The concentration of contaminants and the occurrence of extra

toxins should not exceed the tolerance of the plants used. It is not easy to choose

plants that effectively remove various impurities. In applying this technique, these

limitations and the likelihood of these pollutants entering the food chain should be

considered.


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Table 3. Benefits and limitations of the phytoremediation technology of compounds

(Dhanwal et al., 2017; Ashraf et al., 2019)

Benefits Limitations

Application

It can be employed in situ, i.e., on-site removal of

contaminants, whether within the soil, water, or

groundwater

It is mainly applicable to the top layer of the soil

and mine tailings

It is very effective at sites where low amount/toxic

contaminants are present

It offers limited applicability to diverse kinds of

wastes, especially with high-level toxicity wastes

Cost factor time factor

It offers lower labor expenditures and reduced cost

in operations

Plant deaths may occur in highly toxic sites which

could increase the cost of the process

It comes with low investment cost and minimal

equipment requirement (constitutes substantial

savings)

Mostly there is incomplete removal of contaminants

with long-term low performance

Contaminants can be recovered from the plant

tissues and offer an opportunity for

commercialization

Good cultivation practices and maintenance is

required to avoid accidents

Performance

It can be used for remediating soils that are

nonproductive for agricultural purposes

The effectiveness of this remediation process is

affected by seasonal factors

It has the potential to treat sites polluted with more

than one type of pollutant

A good considerate of the performance and

physiological changes of plants in response to

different varieties of wastes is needed

Impact on the environment and population

It is aesthetically pleasing and widely accepted by

the public community

There is the possibility of bioaccumulation of

pollutants in the food chain

It can reduce erosion of soils, especially thinner

inorganic soils

There is the possibility of introduction and

spreading of undesirable invasive species of plants

It is nondestructive, nonintrusive, highly

biologically active, therefore have a very low

environmental impact on soil and water

Proper discarding of plant matter is required with

proper risk assessment

It reduces leaching of particulate substance and

spreading of toxicants

Interaction of phytotransformation, phytoextraction, and phytovolatilization

technologies

Phytotransformation, otherwise known as phytodegradation, as aforementioned is the

decomposition of organic contaminants released by plants: the action of compounds

(like enzymes) is produced by plants (Saleem, 2016). There is the degradation of

organic pollutants into simple compounds that integrate within plant tissues to

encourage plant growth. Restoration of sites by phytotransformation relies on the direct

uptake of contaminants via a medium and buildup in the plant (Abdullah et al., 2020).

Plants absorb hydrocarbons and extra complex organic molecules and then metabolize

or mineralize them through sunlight-driven chemical reactions (Schwitzguébel, 2017).

For instance, some enzymes can break down waste from ammunition (explosives),

chlorinated solvents, or herbicides. This technology can similarly be used to get rid of

noxious waste from petrochemical and storage sites, fuel leakages, leachate stacking,


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and agrochemicals (Kumar et al., 2018). To successfully implement this technique, the

transformed compounds accumulated in the plant must be non-toxic or less harmful

than the parent compound. Hybrid Poplars are proven in research to convert TCE into

trichloroethanol, dichloroacetic acid, and trichloroacetic acid which is partially

mineralized into CO2 (Leung et al., 2019; Feng et al., 2017). Here, organic pollutants

and heavy metals are being investigated too. Soil conditioning, including chelating

agents, may be needed to break the compounds that connect pollutants with soil

particles to encourage their uptake by plants (Verma, 2017; Franchi and Petruzzelli,

2017). In some cases, plant transformation is used in cooperation with other restoration

techniques or as an improving technique (Mustapha and Lens, 2018).

The discharge of volatile contaminants into the atmosphere via plant leaves refers to

phytovolatilization as previously described and is a form of phytotransformation

(Limmer and Burken, 2016). Even if the discharge of contaminants into the atmosphere

might not attain the full clean-up goal, phytovolatilization treatment may be ideal merits

to the long-term properties of the soil and the danger of groundwater contamination

(Limmer and Burken, 2016). In literature, phytovolatilization is generally considered

beneficial because it usually dilutes atmospheric pollutants and undergoes

photochemical degradation. In this technology plants is not just a research area of

traditional organic pollutants but also an important research area for other contaminants

that occur naturally in the soil and roots of plants. Phytovolatilization of many inorganic

and organic contaminations in the natural and agricultural environment is usually

observed. Studies by Arya et al. (2017) on the phytovolatilization of pollutants have

shed light on ways by which many pollutants are evaporated from plants. In

phytovolatilization, certain organic contaminants become volatile in plants and then

become evaporated. Compounds having a low octanol-air partition coefficient (log Koa

b 5) are reported as been more volatile in plants (Limmer and Burken, 2016). Research

conducted by Nwaichi et al. (2015) showed that plant can promote the migration and/or

degradation of carbon monoxide in the soil; just like Fibrristylis littoralis which is

employed for the biological regeneration of agricultural soil polluted by crude oil (up to

92% PAH, shelf life 90 days). Moreover, compounds that cannot be transferred to

plants owing to being hydrophobic can still be absolved in plant tissues during particle

deposition or air distribution over the soil (Limmer and Burken, 2016). But, the relative

prominence of various remediation techniques remains unclear, especially for less

studied connections (Limmer and Burken, 2016).

Phytoextraction interactions with phytovolatilization are two methods of removing

organic contaminants and detoxification in agricultural soils. It has been reviewed in

this paper that phytoextraction removes soil contaminants by concentrating them on

parts of the harvested plant. Previous research studies (Izinyon and Seghosime, 2013;

Sun et al., 2018) have unraveled Zucchini (Cucurbita pepo) as a good organic

compound storage medium for agricultural land. The mechanistic interaction of

phytoremediation involving phytotransformation, phytoextraction, and

phytovolatilization technologies is illustrated in Figure 2.

Future perspectives in biotechnological enhancement for phytoremediation

Recent studies have offered the physiological and molecular technique of

remediation to improve remediation and the cleanup of the agricultural and natural

environment. Food waste being used has also helped to control the proper management


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of food/vegetables/fruit. This gives plants the natural ability to absorb, decompose, or

concentrate pollutants from soil, water, and air. Contaminants and toxic metals are the

core objectives in phytoremediation

Figure 2. Mechanistic overview of 3-phase phytoremediation process

Plant-microbial assisted interaction

Plant and rhizosphere microorganisms are generally employed to remediate

contaminated land. Under this mechanism, plants work together to promote soil

regeneration through the action of roots and soil microorganisms. Microorganisms

utilize root metabolites (secretions), which in turn allow plants to benefit from microbial

processing/mineral dissolution (Asemoloye et al., 2019). Plants can absorb pollutants

from the soil and transfer them to themselves, while microorganisms mainly decompose

pollutants. With the introduction of transgenic techniques, microorganisms and plants

can progress to better degrade pollutant. Nonetheless, the use of inherently improved

organisms in many countries, different political and moral issues, and related legal

power limits the effectiveness of the application. It is well known that microorganisms

play key roles in the decomposition of toxic substances as they help in the elimination

of undesirable molecules in the environment (Maier and Gentry, 2015). As

microbiology evolves, biologists hire the smallest living things on earth to clean and

remediate highly oil-contaminated ecosystems. Undoubtedly, microbes are a viable and

unused resource for novel environmental biotechnologies (Gaur et al., 2018). Plant

growth in a contaminated field often improves soil quality by increasing microbial

populations and organism diversity, plus adding organic matter to the soil. Biological


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treatment (mainly microbial decomposition) is a natural way of eradicating

contaminants by decomposing contaminated nutrients. Soil microorganisms can be

tested aerobically (aerobic biodegradation) or anaerobically (aerobic biodegradation).

Organic compounds through biological treatment have been effectively performed on

agricultural land under natural conditions (Girma, 2015; Odukkathil and Vasudevan,

2016). Plant in combination with microbes efficiency can be enhanced by mixing,

aerating, and adding nutrients, for example, heterotrophic bacteria are employed to

clean agricultural soils contaminated with crude oil (Odukkathil and Vasudevan, 2016).

An important condition is the presence of microbes with sufficient metabolic potential.

Microbes can be native to the infected area (biostimulation) or insulated from the

subsequent area (bio-augmentation) (Varjani and Upasani, 2019; Salimizadeh et al.,

2018). If these microorganisms are present (Nishiwaki et al., 2018), the biodegradation

of contaminants can be sustained by providing sufficient nutrients, moisture, and

oxygen. Other variables, for example, salinity are usually out of control. The destructive

potential of microorganisms for various pollutants has been previously reported (Mishra

et al., 2020; Chen et al., 2019), namely; hydrocarbons, pesticides, and furans for many

compounds, including PCBs, PAHs, DDT, and others. The application of the pesticide

in the soil can cause problems because according to Abatenh et al. (2017), biological

methods are appropriate for conditions where the pesticide is not harmful to

microorganisms and plants employed for remediation. Compared to other biological

methods, microbiological re-purification is considered to be the most effective method,

but high molecular weight hydrocarbons, with low adsorption and solubility, limit their

availability to microorganisms (Koshlaf and Ball, 2017). Microbe-assisted

phytoremediation can be also carried out by stimulation via inoculation with pesticides

degrading microorganisms (Mitton et al., 2012). It is reported that microorganisms can

secrete enormous amounts of surfactants and enzymes, hence it is possible for pesticides

to undergo extracellular degradation due to these secreted enzymes (Sharma et al.,

2018; el Zahar Haichar et al., 2014).

A study by Balcom et al. (2016) reported the potency of microorganisms to

metabolize micropollutants such as xenobiotics during the process of wastewater

treatment.

These include the concentration of contaminants and chemical nature, the

physicochemical properties of the surroundings, and their availability to

microorganisms (Koshlaf and Ball, 2017; Bharathi et al., 2017). Due to various factors,

monitoring and improving the biological treatment procedure is a challenging system.

These factors consist of the microbial populace that can decompose the contaminant and

environmental factors (temperature, soil type, pH, nutrient uptake, and oxygen or other

electrons) present (Abatenh et al., 2017; Kothe, 2015). The non-existence of

information concerning the effects of several environmental factors on the proportion

and the extent of biodegradation creates uncertainty.

Nano-phytoremediation enhancement

Nano-phytoremediation involves a combined application of nanotechnology and

phytoremediation to decontaminate the environment. Nanotechnology improves the

efficacy of phytoremediation. Nanotechnology can provide an environmentally friendly

alternative to remediation and management without harming nature. Nanoparticles are

used to remediate soil and water contaminated by heavy metals, organic and inorganic

pollutants (Srivastav et al., 2018). A variety of plants, bacteria, and fungi in studies


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(Mallikarjunaiah et al., 2020) also enhance their ability to accumulate very high

concentrations of metals, which are also recognized as hyperaccumulators. Plant species

used in nano-plant engineering cleaning (nano-phytoremediation) technology to

decontaminate contaminated soil include the application of phytoremediation

techniques (for instance, phytodegradation, phytoextraction, phytostabilization) and

nanoparticle.

Due to the high efficiency of pollutants remediated by plants using this technology,

numerous reports (Srivastav et al., 2018) have shown that various

nanoparticles/nanomaterials significantly detoxify or restore contaminated soil from

organic, inorganic, and heavy metal pollutants. Nano zerovalent iron (nZVI), bimetallic

nanoparticles (Pd/Fe) and magnetite nanoparticles (nFe3O4), according to the literature

(Alonso et al., 2018) may rapidly decompose organic pollutants such as atrazine,

chlorpyrifos, trichloroethylene (TCE), pentachlorophenol, pyrene, polychlorinated

biphenyls, lindane, 2,4-Dinitrotoluene and ibuprofen from the contaminated soil

environment.

Studies by Pillai and Kottekottil (2016) reveals that plant species (Cymbopogon

citratus) and nanoparticle (nZVIs) can be used to remediate Endosulfan pollutant at an

efficiency rate of 86.16 ± 0.09 (%). Souri and coworkers (Souri et al., 2017) also

reported that Arsenic (As) pollutant was removed using plant (Isatis cappadocica) and

nanoparticle (SANPs) at 705 ppm and 1188 ppm accumulate in roots and shoots,

respectively. A recent study by Ma and Wang (Ma and Wang, 2018) proves that 82% of

Trichloroethylene pollutants can be remediated by a combination of Fullerene (nC60)

and Populus deltoids from the environment. The success of phytoremediation, or more

precisely the phytoextraction, depends on the hyperaccumulator specific to the

particular pollutant. Preferably, researchers (Srivastav et al., 2018) suggest that the

nano-phytoremediation must be able to accumulate excess contaminants (organic,

inorganic, and heavy metals), specifically in the aboveground part (sinking potential). It

ought to be a fast-growing plant with high biomass (growth and productivity). The use

of selected nanoparticles significantly increased plant growth and treatment with nano

augmentations improves the efficiency of phytoremediation and significantly removed

pollutants from the soil (Pillai and Kottekottil, 2016).

Despite the many benefits, we obtain from this combination of technology; research

on phytoremediation using nanomaterials is very scanty. So far, only microcosm studies

(Simonin and Richaume, 2015) have been carried out, so future studies need to use

more realistic studies and a better understanding of field practicals is needed.

Genetic engineering enhancement

Genetically improved technology creates effective means of plants and microbes

combined with systems of bacterial-plant remediation. Combined cultivation of

contaminant sources and individual microbes, whole cells, living plants and plant

wastes, food, agriculture, and forestry wastes plays key roles in reducing the content of

heavy metals in mining locations to bring a good return to agriculture or the natural

environment. Similarly, genetic manipulations of plant hosts in microbial communities

can improve phytoremediation capabilities (Tripathi et al., 2020). This includes

isolation of bacteria and gene integration that direct the production of specific enzymes

thus resulting in degradation of pollutants using the transport of modified organisms and

an increase in the number of responsible microbial species in the host system.

Biotechnology (genetic engineering) from a previous research study (Dhanwal et al.,


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2017) can be engaged to attach one or more active accumulator genes from higher

plants into smaller plants, thereby increasing the final biomass. Novel catalytic enzymes

can be used in present biotechnology for soil remediation; new microorganisms can be a

replica with precise degrading genes for active hydrocarbon degradation (Asemoloye et

al., 2019).

Currently, genetically improved plant species are produced using genetic engineering

techniques and are used for a plant in remediating soils contaminated with methyl

mercury, a neurotoxic agent (Dhanwal et al., 2017). Conversely, modern

biotechnological advances, for instance, the new omic revolution, the emergence of

nanotechnology, and the innovation of new catabolic genes, may improve the

competence of bioremediation and its applicability in removing soil pollution. To

identify plant traits that depend on a particular combination of several genes; for

instance, a quantitative map of the trait loci between zinc-tolerant (Arabidopsis helleri)

and non-zinc-tolerant (Arabidopsis lyrate) hybrids identified several genomic regions,

and combining them explains why more than 42% of the plant was tolerance to zinc

(Nahar et al., 2017). Arabidopsis and Transgenic tobacco are some examples of the

genes of transgenic bacteria merB and merA (Dhanwal et al., 2017) that have the

potential to eliminate mercury from the soil. Studies have shown that transgenic plants

combined with bacterial genes can transform the herbicide Simazine into different non-

toxic forms (Azab et al., 2016). Nahar et al. (2017) also reported the Arabidopsis

thaliana AtACR2 gene (encoding arsenic reductase 2) which was cloned and

transformed into the tobacco genome (Nicotiana tabacum). The results obtained showed

that transgenic tobacco has a higher tolerance to arsenic than wild tobacco.

For genetic manipulation, the use of bacteria is employed to assist plant remediation.

Therefore, several strains of bacteria and fungi are known to degrade these compounds

(Gilani et al., 2016). In another study, Gilani et al. (2016) identified 14 species of

Pseudomonas and isolated them from the soil as the organisms that could degrade

chlorpyrifos. Many of these soils have been contaminated with a mixture of pollutants

let us say pesticides (Barchanska et al., 2019). Therefore, preserving the steadiness of

genes transferred to a host is a very difficult task (Tripathi et al., 2020), and there are

signs that recombinant organisms with certain characters often lose their efficacy to

degrade some contaminants.

Improvements in omics technology provide opportunities for isolating and

cultivating such useful microorganisms and studying non-cultivable organisms. The

next-generation of sequencing technology can provide an ideal method for analyzing

microbial communities (Van Dorst et al., 2016). Although the higher nutrient content in

the artificial medium usually restricts oligotrophic bacteria (i.e microorganisms that

require only a small amount of nutrients) and prefers copiotrophs bacteria (i.e.

microorganisms that grow under nutrient-rich conditions), therefore, it is similar to the

new cultivation technology of the natural environment is closing the gap between

culturally dependent methods and culturally independent techniques (Van Dorst et al.,

2016; Ferrari et al., 2011).

Conclusion

Biotechnological integration may provide a crucial step towards the development

viability of the remediation of the ecosystem. However, there is a need for more

research into understanding the mechanisms underlying plant-microbial interactions of


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the rhizosphere. Further exploration is also required to develop the kinetics and

simulations for synergistic degradation procedures and their field applications.

Additionally, developments in bioremediation are expected to focus on ways to provide

conditions that promote plant growth and microbial activity in the soil to enable

bioavailability and degradation of organic and inorganic compounds.

Since phytoremediation research is actually interdisciplinary, we recommend the

following:

a) Plant breeders, biotechnologists, physiologists, agronomists, soil scientists,

biochemists, and environmentalists must work together to produce robust

methods to develop genetically modified plants and enhance the potential of

existing plants for better contaminant control.

b) In transgenic research on phytoremediation, future research should be feasible

to solve the problem of mixed pollution that occurs in many contaminated

locations to remove mixed or complex pollutants.

c) Future research should also focus on making better use of metabolic diversity,

not only for plants, but also for a better understanding of the complex

interactions between pollutants in the rhizosphere, plant roots, soil, and

microorganisms (bacteria and mycorrhizae).

Acknowledgements. This work was supported by the National Key Research and Development Program

of China (2016YFD0200101), Science and Technology Project of the 13th Five-Year Plan of Jilin

Provincial Department of Education (JJKH20190908KJ), and Natural Science Foundation of Jilin

Province, China (20190201274JC).

Conflict of interests. The authors declare that they have no conflict of interests.

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